This invention generally pertains to bearing members adapted to support beams or decks upon piers, foundations, sills, etc., and more particularly pertains to an improved elastomeric bearing structure which, solely through compressive and shear strain or deformation, will accommodate imposed static and dynamic loading, thermal movement, non-parallel loading and the like.
In the construction of large structures, such as a bridge or a building, an important factor which must be taken into consideration is the movement of the individual structural members relative to one another. Such movement can be due to a number of factors, such as the thermal expansion and contraction of the materials being used and also external forces, such as wind, earth movement and the like on the structure along with the static and dynamic loads applied to the members themselves. In a bridge structure, horizontal beams are suspended between spaced vertical supports with the ends of the beams terminating at the supports. In such an application, it is necessary that provision be made for the thermal expansion and contraction of each beam as well as the angular or rotational movement caused by beam deflection from traffic loads on the bridge. The present invention, as herein disclosed, comprises an improved elastomeric bearing for such applications.
The basic concept of supporting bridge beams or the like by means of load bearing elastomeric material is a pertinent application of elastomers as a structural material. The applied unit loads and various movements are compatible with the load bearing and elastic characteristics of the material, while design and fabrication requirements fall readily into accepted practices in the rubber industry. Beam movements are accommodated by rubber deformation, not relative motion. It has been proven that elastomeric bearings may effectively support the various reactions and accommodate the required movements of structures within the load bearing and elastic properties of the material. Considerable cost advantages are obtained and the necessity is eliminated for design of expensive moving parts and their subsequent maintenance.
The design of an elastomeric bearing begins with the understanding that a rubber compression spring is a device by which the gravity forces of a structure are to be balanced by the "memory" of a specific elastomeric compound or its capability to regain its original form. Rubber has this ability to deform and comply to extreme load conditions, and will predictably resist the resulting stress and return to normal upon release of the load.
Toward this end, extensive research has been devoted to study the load deformation characteristics of load bearing rubber. Because it is a complex material, designing to ultimate limits is also somewhat complex. Keeping the spring concept in mind, bearing design is begun on the simple premise that the less the compound has to deform or remember, the better and longer it can function properly. Keeping initial compression deflection and deformation within limits which are low enough to insure against further deformation, or settling, during the life of the structure, becomes the principle ruling criteria.
Probably the most important characteristic of rubber that makes it suitable for use in bridge bearings is the relative ease with which its compression modulus can be altered to meet the designer's needs. The compressive modulus is highly dependent upon the geometrical confinement of the rubber, which has been characterized by the term "Shape Factor" and is defined as the ratio of the effective bearing area under load to the exposed area free to bulge as a result of rubber displacement.
For example, if a bearing receives 500 p.s.i. dead load, and the rubber thickness is such that the perimeter surface area free to bulge is equal to the load area (shape factor of 1), the bearing will compress about 30% of its thickness immediately upon placement of the beam, and with time, will continue to creep or bulge out the sides. However, if the rubber thickness is reduced until this bulge area is only one-sixth the load area (shape factor of 6), deformation will then be less than 5% of thickness and subsequent creep or progressive deformation well be inconsequential or non-existent.
It should be noted, that in shape factors above 6, durometer change has no significant effect upon compressive deflection; a valid indication that a degree of rubber confinement has been reached where compression stability is permanent. This shape factor versus compression strain relationship, therefore, is simply a precise statement of the correct degree of rubber confinement required for the load ranges involved.
To summarize these load bearing design procedures, two principal controls are used: (1) a correct number of square inches in the plan area to support a given load, and (2) an effective thickness allowed for bulge which is correctly proportioned to the plan area in order to eliminate failure from settling or permanent deformation.
It should be noted that the shape factor effect assumes that bearings are restricted from any lateral movement between load surfaces by way of chemically bonding the elastomer to sole plates or having the elastomer in contact with a rough surface exhibiting a high frictional coefficient, such as concrete and the like. A simple unbonded bearing will function satisfactorily only if the load surfaces are permanently clean and dry and no outward surface creep between the load surfaces and the surfaces of the bearing is possible. In terms of functional longevity, the compression or settling life of an unbonded bearing depends substantially on the ability of the coefficient of friction between the bearing and the beam to be sufficiently high to prevent spreading. In applications of the present invention, there is intended to be substantially no slippage or creep between surfaces of the elastomeric body surfaces and the loading surfaces when reliance is placed on frictional engagement.
Designing the bearing to accommodate the various movements of the beam is a matter of selecting the thickness as a function of the amount of lateral movement anticipated. This comparison is necessary to determine. the shear strain in the rubber. In order to minimize high shear loads being transmitted to the pier or foundation, the elastomeric mass should not be extended laterally more than 25% of its thickness each way while under load, for example, as an empirically sound design rule.
As known, the rubber mass moves equally well in any direction. Since allowable shear travel will be 0-25%, for example, total allowable movement is half the thickness of the bearing. Conversely, the bearing thickness must be twice the expected total movement. Although it is unlikely that beams will be installed at temperatures representing the exact midpoint of their expansion, any additional strain or deformation should fall well below the ultimate permissable shear strain.
Assume that a beam or deck proves to have a potential horizontal movement equal to the thickness of the rubber. The basic bearing of a selected shape factor then provides only half the required travel capacity because of its thickness. In the prior art, another identical bearing has been positioned on top to gain the required thickness and both are bonded to a common steel plate at their common load surfaces. This double bearing still has the same load carrying capacity as the single basic bearing, but the accumulated lateral travel capacity of the two bearings now equals the expected beam movement. However, the common steel plate adds thickness to the composite bearing which does not contribute to such lateral travel capacity as permitted by the present invention.
The flexural or bending of beams under load causes a rotating movement of the upper surface of the bearing. The rotating load effect on the rubber is different from the effect of vertical dead beam load for several reasons. Dead load compression, evenly applied, causes transfer of rubber mass into the side bulge volume. The live rotating load causes an increase in bulge on that side of the bearing facing the beam length with a corresponding reduction on the opposite face. The actual difference in effect on the rubber is a uniform outward mass movement in the case of dead load and a non uniform mass transfer during bearing rotation.
Still another load effect is due to permanent non-parallelism of load surfaces. In this instance, side transfer is permanent and, over a fairly wide latitude, does not materially reduce the load carrying capacity of the bearing. While rubber has the ability to conform to a new permanent working position, care must be used not to exceed the "memory" of the compound.